Pyrazolotriazoles, such as those described herein are useful magenta couplers for photographic products; however, they are difficult to synthesize. Only a few synthetic routes are known. One of the preferred synthetic routes involves preparation of the triazolothiadiazines (III) and subsequent desulfurization reaction to give the pyrazolotriazoles (IV). U.S. Pat. No. 5,055,586 of Kim et al. fully describes the state of the art and its contents are incorporated herein by reference. The triazolothiadiazines (III) can be prepared in two ways: the first, by reaction of 4-amino-5-mercapto-3-substituted-1,2,4-triazoles with alpha-haloketones, or the second, by reaction of 2-hydrazino-5-substituted-1,3,4-thiadiazines with acyl halides and subsequent dehydrative ring closure. Both the triazoles and the thiadiazines are readily available from thiocarbohydrazide.
The following schematic shows the known process for the further treatment of the thiadiazine. In the known process, the first step is a ring contraction reaction of (III) by heating in acetic anhydride to give 1-acetyl-7-acetylthio-3,6-disubstituted-1H-pyrazolo[5,1-c]-1,2,4-triazoles (I); the second step is hydrolysis of acetyl groups and desulfurization at the same time with hydrochloric acid to give the desired compound (IV); the third step, to obtain a halogenated material, is to halogenate (IV) to obtain (II). ##STR1##
Thus the known process provides for heating in an anhydride to give 1-acyl-7-acylthio-3,6-disubstituted-1H-pyrazolo[5,1-c]-1,2,4-triazoles (I). Hydrolysis of acyl groups and desulfurization at the same time with concentrated hydrochloric acid gives 3,6-disubstituted-1H-pyrazolo-[5,1-c]-1,2,4-triazoles (IV). Subsequent halogenation of the Compound IV gives the 3,6-disubstituted-7-halo-1H-pyrazolo[5,1-c]-1,2,4-triazoles (II).
The hydrolysis and desulfurization reaction with hydrochloric acid, however, generates many sulfur-containing impurities as by-products as more fully disclosed in U.S. Pat. No. 5,055,586. Moreover, in the mentioned patent, it is proposed to overcome the problems related to by-product generation by the inclusion of hypophosphorus acid in the hydrolysis step. However, it has been found that the use of hypophosphorus acid results in formation of hydrogen sulfide as a by-product. In view of the gaseous and toxic nature of the hydrogen sulfide and its ill odor, its generation presents an undesirable environmental, safety and health risk requiring extensive precautions and controls.
It is therefore a problem to be solved to provide a process for the desulfurization and halogenation of thioacylated pyrazolotriazole compounds that produces the desired product in good yield and where the sulfur by-products are reduced in quantity, are in readily removable oxidized form, and are substantially free of hydrogen sulfide.